1. Field of the Invention
The present invention relates to a photo signal amplification transmission system for amplifying a photo signal (multi-wavelength signal) constituted of plural multiplexed signals having different wavelengths which are transmitted through an optical fiber cable, and particularly to a control over an amplitude in accordance with the number of multiplexed wavelengths.
2. Description of the Prior Art
As is well known, when a photo signal is transmitted over a long distance by using an optical fiber cable, a failure occurs on a receiving side even if output power is. too low or too high. Therefore, an optimum output power is required.
Therefore, in the long-distance transmission of the photo signal by means of the optical fiber cable, in general a relay amplifying device is installed at each relay point, so that amplification is performed to obtain the optimum output power.
FIG. 1 is a block diagram showing a conventional photo signal amplification transmission system.
In FIG. 1, numeral 31 denotes a WDM (wavelength division multiplex) photo coupler for combining an input signal from IN and an activating laser beam; 32 denotes an erbium doped fiber (EDF) for amplifying a photo signal when an activating light is injected; 33 denotes an output monitoring photo coupler; 34 denotes an activating laser beam source (referred to as the activating LD); 35 denotes an automatic output power control (APC) circuit; and 36 denotes an output monitoring photo detector(PD).
In the conventional photo signal amplification transmission system, for example at a relay point, as shown in FIG. 1, the EDF 32 and the activating LD 34 constitute an optical fiber amplifier. An output level is monitored by the output monitoring photo coupler 33. The monitored output level is compared with a predetermined optimum value (fixed value). Then, the activating LD34 is feedback-controlled by the APC 35 in such a manner that the output level reaches the optimum value.
In the conventional photo signal amplification transmission system, since the output power is generally controlled to reach the predetermined optimum value (fixed value), a problem occurs when the photo signal (multi-wavelength signal) constituted of plural multiplexed signals having different wavelengths is transmitted.
Specifically, in the conventional system, even the multi-wavelength signal is controlled as one signal in such a manner that it reaches the predetermined fixed value. Therefore, in the case of one wavelength (one signal), the output power is largely varied in accordance with the number of multiplexed wavelengths.
For example, as the number of multiplexed wavelengths is increased, the light output power for one wavelength is decreased. Therefore, when multiple signals (wavelengths) are multiplexed, the light output power for one wavelength is weakened to a degree to which a dynamic range of a receiver cannot be followed. As a result, an error in transmission of the photo signal or another problem arises.